This command can be used to change a layer's z-position. The z-position
of the layer is its position in the hierarchy of layers. The first (i.e.
backmost) layer has a z-position of 1, the last (i.e. frontmost) layer's
z-position is equal to the number of layers currently present. You need
to pass the new desired z-position for the specified layer to this function.
The layer will then assume exactly this z-position, existing layers that
are on or after this z-position will be shifted down. To move a layer all
the way to the front (i.e. highest z-position), you can pass the special
value 0 for the zpos argument. To move a layer all the way to the back,
specify 1 in the zpos argument.
You can also pass a layer name in the zpos argument. In that case,
the layer specified in the first argument will assume the z-position of
the layer in the second argument.